You can post either a card or letter. Both are acceptable.
You can write to The Queen to the below address :
Her Majesty The Queen,
Buckingham Palace,
London, SW1A 1AA,
United Kingdom.
Letter Format
Type I
Madam, {Opening, Salutations}
I have the honour to inform Your Majesty ... [body of letter]
{Closing}
I have the honour to remain, Madam, Your Majesty's faithful and obedient subject.
[signature]
[your name below signature]
Type II
Madam, {Opening, Salutations}
With my humble duty ... [body of letter}
{Closing}
I have the honour to be, Madam, Your Majesty's most humble and obedient servant.
[Signature]
[Your name below signature]
You can write to HRH Prince William to the below address :
HRH Prince William of Wales,
St. James Palace,
London, SW1A 1BS,
United Kingdom.
Letter Format
Type I
Sir, {Opening, Salutations}
[Body of letter]
{Closing}
I have the honour to remain, Sir, Your Royal Highness' most humble and obedient servant.
[Signature]
[Your name below signature]
Type II
Sir, {Opening, Salutations}
[Body of letter]
{Closing}
I have the honour to be, Sir, Your Royal Highness' obedient servant.
[Signature]
[Your name below signature]